1. Field of the Invention
The present invention relates to a superconducting magnetoresistive element made from a superconducting material having weak couplings at grain boundaries, and a method of fabricating the same.
2. Description of the Prior Art
FIG. 8 shows a conventional superconducting magnetoresistive element of the above-mentioned type. This superconducting magnetoresistive element has a Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x superconducting body 101 having weak couplings at grain boundaries, namely, equivalents of Josephson junctions. At both ends of the superconducting body 101, electrodes 102 and 103 are provided. The superconducting body 101 is fabricated as follows. After a mixture of the starting powders is calcined at 900.degree. C., the product is pulverized and remixed. The powders remixed are pressed into a thin plate and the thin plate is finally calcined at 1000.degree. C., becoming the superconducting body having the weak couplings.
FIG. 7 shows a magnetoresistive characteristic of the superconducting magnetoresistive element of FIG. 8. As understood from the graph of FIG. 7, the superconducting magnetoresistive element yields a breakage of the superconductivity of weak couplings at an extremely weak magnetic field (a few gausses), allowing an electric resistance to appear, and the electric resistance abruptly increases with increasing strength of the applied magnetic field. Therefore, the superconducting magnetoresistive element has high sensitivities at weak magnetic fields, as compared with semiconductor magnetoresistive elements.
However, the conventional superconducting magnetoresistive element has the following problem. Since the element has only a single superconducting body 101 wherein the weak couplings or Josephson junctions are disorderly arranged, a loud noise is generated, so that the S/N (signal-to-noise) ratio is low.
The superconducting body having weak couplings at grain boundaries has a low critical current density. It is known that a higher S/N (signal-to-noise) ratio can be offered if a plurality of portions having a low critical current density (referred to as weak-coupling portions hereinbelow) like the superconducting body 101 are provided in a manner to alternate with a plurality of superconducting portions which have such a high critical current density that they hold a zero electric resistance to any strengths of magnetic field within a measuring range, so that those weak-coupling and superconducting portions are connected in series. But, since the conventional fabrication method includes the calcining steps, it is extremely difficult to obtain such an alternate arrangement of the weak-coupling portions having a low critical current density and the superconducting portions having a high critical current density. As a result, with the conventional fabrication method, it is impossible to increase the S/N ratio of the superconducting magnetoresistive element.